Flap Speeds - 4 place Bravo

[Battson]

50 degrees of flaps shouldn't be used for any normal approach-to-landing.

Who taught you to fly.... Jokes aside, I am always concerned when I see people landing with less than full flap. It greatly increases difficulty, risk factors, and heightens the risk of a ground loop. Aside from special situations like crosswind landings / emergencies / training.

For instance, when was the last time you saw a commercial plane landing with half flap?

[nborer]

Lol. I'll agree with you in general, so I apologize in advance if this sounds argumentative. You have a lot more trophies than I do when it comes to landing where you want to.

In a response on the previous post, I noted that the C172N has 10 degrees more of flap than the C172P. That's because they upped the gross weight of the 172P by 100 lbs. Our flying club recently adopted an STC for our N model that increased the gross weight - it basically involves a lip around the oil cooler (we already had it as a late-model N) and a reduction in max flap travel from 40 to 30 degrees. I was curious about this, and what it really came down to was the balked landing climb gradient. To meet cert requirements, Part 23 aircraft are supposed to have a balked landing climb gradient of at least 3 percent. That's with the aircraft in the landing configuration, which includes all approved flap settings for landing. Both the N and P model 172s have a 160hp engine, and I suspect that in some conditions they couldn't meet that balked landing climb requirement at the increased weight with 40 degrees of flaps, so away it went.

In the experimental world, we have the luxury of being able to pick our engines. I have yet to create a performance model for the Bearhawk, but it's on the list. (Has anyone come up with a drag polar, both flaps up and down? I can WAG one, but it won't be very good at low speed.) That said, the excess thrust (T-D) from a prop turned by a 180hp engine vs. a 230hp engine is going to be pretty dramatic - T is going to go up roughly proportional to the power, and D isn't changing much... and climb gradient is related to (T-D)/W. Given the very high drag of the Bearhawk flap at higher flap settings, I suspect that some engine choices could result in an anemic climb gradient in a go-around, particularly at max gross weight and a high/hot day.

Commercial planes (particularly big ones) have very complex, very effective flap systems - leading edge slots and multi-segment Fowler flaps that are very effective at increasing the maximum lift coefficient. They are designed to significantly lower the stall speed of the airplane in the landing configuration to get the approach speed (and therefore landing distance) down. It irresponsible for them to not use all that lift - brakes (and overruns) are expensive. That said, the speedbrakes and spoilers don't (generally) come out until the wheels are on the ground.

In the case of the Bearhawk plain flap, it doesn't seem to add much lift beyond 15 degrees, and beyond 30 it's a speedbrake - that is to say that all it does at the higher settings is increase your sink rate, not your lift capability (that experience may change with VGs). That's great for obstructed areas, but I fly out of a Class D airport with a 7000 ft runway. They want me on a 3-deg glideslope in a bomber pattern. I still approach high - I see white on white quite a bit - because if that spinny thing in front of me stops when I'm a mile out, I want to at least make the clearway.

All that said, I land with full flaps in our club planes unless winds dictate otherwise or I'm training. "Full" for the Bearhawk is just a bit more of an extreme than most planes. I've never flown one, and my technique will depend on my experiences if/when I finally get it in the air. I also have initially though I'd put in a 180hp engine, in which case my go-around climb gradient could be a bit anemic, particularly at gross weight. Lots can happen between now and then to change my mind.

[Sir Newton]

I always land in the same configuration regardless of wind conditions or landing weight.

[AKKen07]

For what its worth - we land 737's with "half flap" fairly often. And our max flap setting of 40 is only common when needed to land on shorter strips. Flaps 30 is by far the most common setting with flaps 15 preferred for high DA situations with long runways like Denver, CO. The primary rational for the lesser flap settings is improved performance in a go-around/climb-out, reduced flap wear, and lower airplane noise levels (you might be surprised how important that last point has become). Some pilots who are used to large airports find flaps 40 simply uncomfortable after using flaps 30 99% of the time and so reinforce the flaps 30 standard. I don't know how much of that pertains to a BH but since you mentioned commercial planes...

[Nev]

Is <65mph for 50 deg of flaps the new official guidance for the Model B?

I wrote to Bob and he replied the speeds that I've copied early in this thread. Not certain if that's "official", so to clarify, it's second hand, via me.

nborer you make some very interesting observations via your charts. The one that grabs my attention is that with full flap the effect on lift is minimal, but the effect on drag is large. This makes sense on a Bush/STOL aircraft, for the ability to make steep approaches etc and I think it's probably a large factor in the short landing ability.

My understanding is that drag also obeys the dynamic formula in relation to being proportional to V² and the Bravo model has huge flaps. Hence the inflight loads would rapidly increase proportional to the square of the airspeed. In the previous aircraft I was current on we frequently flew approaches at a reduced flap setting as it lowered the flight loads and maintenance on the flaps (and reduced fuel consumption).

[nborer]

Yes, parasite drag is also proportional to V^2. Paradoxically, for level flight, induced drag is proportional to 1/V^2, hence why we have a "back side of the power curve." The loads we're talking about on the flaps will go up quite a bit with V^2 due to the parasitic effects. My thought/concern was that my plans list 75mph as the limitation for flaps at 50 degrees, which implies to me that he's cleared the structure for those loads at those speeds. If he's now saying that we should only fly 65mph with 50 degrees of flaps, I am curious if that is due to a revision in the structures calculations or if it's more related to experience/flight technique. It doesn't effect me now, but it'll make me think about what I placard/etc. when I get to your stage of the build... many years from now...

[Battson]

In the case of the Bearhawk plain flap, it doesn't seem to add much lift beyond 15 degrees, and beyond 30 it's a speedbrake - that is to say that all it does at the higher settings is increase your sink rate, not your lift capability (that experience may change with VGs).

Well, I must say I think oversimplifying (overthinking?) it - but you clearly know your stuff, so good for you

You can feel it when you land. Drag is required to land an aircraft with maximum safety.

Landing with 25 degrees flap or below, the Bearhawk is not easy or fun to land. You have a higher deck angle meaning visibility is reduced unnecessarily, the plane is slippery, it tends to float and bounce. Control is more difficult. The tail is remarkably heavy and steerage in the three-point attitude is more difficult.

Lift is not the whole story. We are trying to descend, without building airspeed, we need lots of drag. When we reach the ground, we want to stop flying ASAP, floating down the runway in a semi-three point attitude is an accident waiting to happen. Especially if wind is present.

If your engine fails you need to react to the situation before you, wherever you are. I don't buy the argument that landing with half flap somehow improves your odds. Most engine failures happen on take-off anyway. This is for turbines but you get the idea:

image_10081.png


I have to say that my experience doesn't agree with your theoretical data, about 15 degrees creating maximum lift. Experientially, 15 degrees does NOTHING to improve lift. In fact, it does next to nothing at all.
Most of the lift happens around 30 to 40 degrees. The last notch to 52 degrees is pure drag.

[nborer]

I'll take real-world experience over theory any day. If I don't have an experience basis, I'll start with theory. It often misses some nuances. You clearly have the experience here, and it trumps any theory I may spout. I've never flown a Bearhawk, but I'm itching to find out!

Interesting that in your experience the first bit of flap deflection doesn't improve lift. I haven't tried modeling the A wing, but that should be straightforward given that it's a NACA section. One thing I don't have is the location of the hinge point. I can guess the chordwise location of the hinge point, but don't have the vertical position. I could probably WAG something, but if someone has a drawing handy and can let me know how far down vertically the hinge point is from the chord line, it wouldn't take long to model. Otherwise I'll guess something.

There will be some 3D effects - vortex lift from the flap edge - but I can't see this as why higher flap deflections would be so much better. For some crazy low-aspect ratio swept wing, absolutely. One of the local military contractors here flies F-21 Kfirs to "attack" the Navy and Air Force from the airport where we're based. I love watching them on approach to land - that plane ain't going where the nose is pointing at low speed!

As I recall, you have VGs. Do you mind sharing where they are located, approximately? I can trip the flow at a specified location. Did you notice that the first 15 degrees didn't increase lift prior to putting on the VGs?

Regarding an engine failure, the most important thing to do in a single-engine airplane is push to keep (or get to) the best glide and/or min sink airspeed (depending on your situation - usually best glide), and then configure as necessary. The more flap you have, the more you need to push when the spinny thing quits. The planes I currently fly all have electric, slow-acting flaps. I generally don't apply full flaps until I'm sure I can make the clearway if the engine quits. I suspect it's a bit different when you have a Johnson bar - I've only flown two airplanes that have had one (both Pipers). I actually kind of like them, and I see it as a selling point of the Bearhawk. But, I agree, approach to landing is not usually the spot where you need to worry about the engine quitting. Two exceptions - if you're bad a fuel management, or if you are susceptible to carb icing (which won't show up in the jet data). It's pretty humid here, and we've known a few folks that got a wake-up call when they got complacent and forgot that there's a carb heat knob they need out when landing.

[Battson]

If you don't have a set of Bearhawk plans yet, you could buy some?

That will answer your questions about the real wing profile (which are not exactly the same as the NACA profiles), the hinge point, and set you on your way to perhaps flying a Bearhawk of your own.

VGs - use a search engine (Google?) over this forum, you'll find what you're after.

Lift is a practical thing in flight, best measured by checking stall speeds at different flap settings. I've done all the CFD stuff, it's fun but hard to get realistic results.

[nborer]

Thank you. I have a set of plans for a Model B, and was even doing well at making parts until my shop/garage became a storage area for stuff following a massive kitchen flood. Hence, when I'm not working at my job or working on the house, I look over my plans, come up with designs for a 9' brake, conduct analyses, or whatever else. I guess I'm really not all the interested in buying a set of model A plans just to satisfy a curiosity related to a hinge point. I can probably make an educated enough guess for the error bars I'm working with already.

CFD is great but has to be interpreted with a grain of salt, particularly in cases for high-lift. It's not an excuse to eliminate experimentation, but done right, really helps to inform it.

[jaredyates]

I believe Battson also has a little more wing area thanks to extended tips?

[Nev]

My thought/concern was that my plans list 75mph as the limitation for flaps at 50 degrees, which implies to me that he's cleared the structure for those loads at those speeds. If he's now saying that we should only fly 65mph with 50 degrees of flaps, I am curious if that is due to a revision in the structures calculations or if it's more related to experience/flight technique.

I'm currently working on one of the flaps and I'm also very curious about the revision. The B model flaps are truly huge. Yesterday, my mentor and engineer took a look and was very interested in the attachment hardware, particularly the hinges. He expressed a concern of making certain not to overspeed the flaps in case damage occurs.

During flight testing I'll pay attention to the landing characteristics to see if perhaps it makes sense for me to use a lesser flap setting when runway length is not a limiting factor (most of the time).

Another consideration is the go-around scenario. Having 260hp will perhaps make the climb gradient a non issue, but it might be very easy to overspeed the full flap setting, especially with the full prop wash being directed at them.

[svyolo]

There are lots of reasons why airplanes land at less than full flaps, including large commercial A/C. I agree with Nev that 260 hp in a BH covers a lot of those up. Some of those reasons are certified requirements, to meet some performance minimum requirement, some are really idiotic and the local government requires it to reduce noise. Some are driven by bean counters to save .01 percent of fuel burn. Some, in my opinion, actually make aviating less safe.

For me, I always found it best to land one way as often as possible. You can't help but get better at it as time goes by.

[Battson]

He expressed a concern of making certain not to overspeed the flaps in case damage occurs..

Charming, but not very useful. It's amazing how little metal it takes to hold something together. If lifted slowly, one control cable should lift a family car or a Bearhawk. Now compare the hinges to a cable.

I have accidentally flown with 25 degrees flap at cruise speed for fractions of an hour, on a couple of occasions. It isn't a problem. I have, of necessity in emergencies, considerably oversped with full flaps. Again, no issue.

I think it's a lot about usability rather than structural load. I believe Bob sets the flap speeds based on the handle and the quadrant, and how hard the pilot has to pull. I understand that's the first thing that is expected to break is the welds in the bottom of the handle, or break your arm more's the point...

Another consideration is the go-around scenario. Having 260hp will perhaps make the climb gradient a non issue, but it might be very easy to overspeed the full flap setting, especially with the full prop wash being directed at them.

I don't tend to apply full power with full flap like you would in a cub or 172, unless I have a real emergency - in which case you're so slow and needing to climb so overspeed is not a concern - almost always the opposite, stall speed. If I am going around in a controlled way with no rush, I go up to about 65% and then bleed off some flap and re-trim, then increase power again and climb.

[robcaldwell]

I have to agree. Unless there are obstacles, on my go arounds I apply enough power to stop the descent, maintain altitude and controlled flight. I clean up the airplane then gradually add more power to begin the climb. The black knob on my 540 doesn't move forward very fast.

[nborer]

The limiting factor for flap speeds in certified aircraft is usually related to load at that speed at maximum load factor. If I recall, the limit load factor for most aircraft with full flaps is 2g, meaning they're designed for 3g assuming a factor of safety of 1.5. So, if it's not turbulent or you're not exceeding 60 degrees of bank in a turn, you won't start cutting into the structural margin.

The C172s in our club have exhibited some wear on the flap tracks, and one of them had cracking that required track replacement (not fun and expensive). That's usually the long-term effect of continual flap overspeed events - fatigue and wear, rather than sudden failure.

Point being, there should be some decent margin there. That said, (I think I read it on this forum, and I love this quote): "margin belongs to the designer." Meaning that it's there for the little oopsies that sometimes happen, not something anyone should normally count on in operations.

[Nev]

I was looking at the flap speeds listed earlier, and I noted that the 50-degree setting was dropped from 75 mph (as per my plans) to 65 mph. Certified aircraft (at least for this type of airplane) are supposed to pick a reference approach speed that is at least 1.3x the stall speed in the landing config. If we have a 53 mph stall speed, 1.3x this is about 69 mph - above the stated 65 mph flap speed. I suppose no one really approaches with 50 degrees of flaps unless you're deep into bush territory (in which case an approach speed that is 1.3x higher than stall speed is silly), but as normal procedures go, 50 degrees of flaps shouldn't be used for any normal approach-to-landing. I don't know what speeds folks use, and I suppose lots of folks don't fly at gross weight or even close, but it's something I'll now add to my eventual flight manual - if I can ever stop playing with XFOIL and get my shop back (still waiting to clear it out after it became a "stuff" repository after our emergency house repair that turned into a giant remodel).

Is <65mph for 50 deg of flaps the new official guidance for the Model B? I may have missed it; I know some good stuff shows up in the engineering change notices. I'll need to be sure to log that one.

Just reading some of your posts here - very interesting, and most of your assumptions are very close to my experience to date. I gather you probably have a lot of experience and knowledge in these areas.

On my Bearhawk, Flaps 1 & 2 appear to increase lift a small amount but not drag so much. Flaps 3 is a good all-rounder and Flaps 4 increases drag a lot - excellent for short landings at lighter weights. When combined with power on approach, flaps 3&4 reduce the stall speed significantly and at very low airspeeds result in a higher body angle. At altitude, with 20" MAP I've seen a pitch attitude of up to 28°.

Because the F4 speed restriction is only 55 KIAS, it becomes unusable at heavier landing weights. I frequently land using F3, particularly if carrying 3 POB or more. It allows me to carry a few extra knots on approach and gives better all round landing results - for a slightly longer landing distance. As others have mentioned already, it's very common for the air transport aircraft to use a reduced flap setting for fuel economy, lower flap wear, and where Landing Climb performance is an issue.

I placed my VG's at 5% MAC. (This was what the manufacturer recommended). My F4 power on stall speed at 2200 lbs is 38 KIAS (37 CAS) vs 40 KIAS power off - my IAS indicates higher at these high AoA's. Realistically I'm finding that when landing on the shorter rougher airstrips, I'm limiting my landing weight to 2200 lbs. This allows use of F4 and generally gives a very good landing performance. I use an approach speed typically of 50-55 KIAS depending on weight. This equates roughly to VS1.3. On short approach when at light weights the speed will commonly reduce to 42-45 KIAS.

[Battson]

Because the F4 speed restriction is only 55 KIAS, it becomes unusable at heavier landing weights.

How do you figure?

I have used full flap on almost every landing for a decade, so it cannot be "unusable"!

[Nev]

How do you figure?

I have used full flap on almost every landing for a decade, so it cannot be "unusable"!

Well several reasons :

My B model plans state 50° - 75 mph. I clarified with Bob that this was an error, and should be 65 mph (56 kts, although I use 55 for simplicity).

Although the plans don't make mention whether the flap limit speeds are IAS or CAS, I assume them to be CAS to account for position error, otherwise it doesn't really mean much. nborer gives an excellent explanation in 15.1 of this thread where he states : "One culprit is IAS vs. CAS/EAS vs. TAS. At low speeds, pitot-static systems notoriously under-indicate airspeed." He uses the example from a C172 POH of a typical 13 kt spread at the stall between IAS and CAS at F30.

So in order to compare speeds we need to quantify the position error and use CAS.

On my own aircraft, when I'm flying 55 KIAS the CAS is (unusually) 1 knot lower (so yep, I could pull F4). However on many aircraft when flying at low speed, the CAS is often higher by 5-10 kts. So it would be easy to be flying an approach IAS of say 48 KIAS, but have a CAS that exceeds the flap limit speed.

I typically use an approach speed of 50-55 KIAS depending on weight. So at heavier weights I find myself right on the F4 limit speed and therefore use F3.

[Nev]

Is <65mph for 50 deg of flaps the new official guidance for the Model B? I may have missed it; I know some good stuff shows up in the engineering change notices. I'll need to be sure to log that one.

I'm not sure about this. I just noticed that my plans had 3 flap speeds whereas the aircraft had 4 flap settings, so I called Bob to discuss. His reply was that the F4 limit speed should be 65 mph.

[Battson]

Well several reasons :

I typically use an approach speed of 50-55 KIAS depending on weight. So at heavier weights I find myself right on the F4 limit speed and therefore use F3.

The crux of the discussion is around what speed the Bearhawk 4-place will fly safely, vs. pilots preference. Flown with care in the STOL mindset, approach at 1.1 Vso is certainly realistic, and for most 1.2 Vso should be very safe. Speed control should be within 1 or 2 knots on a stabilised approach profile on a fair weather day. I suppose 1.3 Vso is probably appropriate for a fast twin...? Not my area of expertise!

For a Bearhawk 4-place, say the stall speed is 44 kts @ 2500lbs (conservatively, it is probably less):
At 1.1 Vso the approach speed is 48 kts, full flap is usable.
At 1.2 Vso the approach speed is 53 kts, full flap is usable.
At 1.3 Vso the approach speed is 58 kts - you should have bought a C182...

I don't think CAS vs IAS is important at these airspeeds, unless an aircraft has huge instrument + position errors. At these speeds, it should be a non-issue. Having watching the GS for years, my CAS / IAS must be within a knot until I get below 42 KIAS - below that the errors ramp up quickly. It seems like we're the same in that regard, albeit yours reads over and mine reads under.

Before we get too scientific, consider the margin for error around the 55 kts full flap speed....
Also consider, are you actually achieving full flap...?

[AZBearhawk272]

Not meaning to derail this thread, wanting to open up the texture a bit. A purpose directed stabilized approach
is needed particularly for back country operations. It is derived from aircraft specific performance and applied based on runway environment, weather conditions and pilot skill and confidence.
I cannot apply one condition from my experience where I plan or reflect based on what Vs 1.x was used.
It is always a performance attribute that gives me feedback on my aircraft control and environmental variables. Often my go, no go decision is from fly through, turbulence, obstacles, respiration, stick grip,
raised neck hair, or the observed glide path.
A high priority is how does the movie out the windscreen look? Airspeed is a historical look at the past when seconds matter. If my glide slope, descent rate and aiming point converge in an unsatisfactory conclusion not sure what the Vs 1.x matters if the mission failed. Or asked another way, how happy would I be walking away from a pile of rubble knowing I maintained Vs x 1.2?

Kevin D # 272
KCHD
Marking calendar days till COVID goes away!

[Battson]

Completely agree - 1.x Vs is used for discussion purposes only.
Practically, a backcountry pilot seldom knows what airspeed the approach was flown at, too busy flying the approach.
I would hasten to add, that a slight overspeed on the Bearhawk flaps has no practical consequences for the airframe, so monitoring speed to ensure Vf4 isn't exceeded, isn't a flight safety consideration when compared to flying the aircraft.